1. Field of the Invention
The present invention relates generally to a method and apparatus for acoustically analyzing the ratios of a mixture of gases consisting essentially of two known gases. Particularly, the present invention is related to such a method and apparatus that utilizes transmission of ultrasound through each of a reference and sample gas, and that utilizes the ringing signal in the electronic receiver circuitry which receives the ultrasound signal transmitted through the reference gas, as a clock.
2. Description of the Related Art
There is a recognized need in both industry and medicine for a reasonably accurate, modestly priced, stable, continuous monitor of binary (two-gas) mixtures. Although numerous types of instruments are commercially available to perform analyses of gas mixtures, most are expensive and complex because they are designed to laboratory standards for monitoring specific gases. In many cases, the fragile nature of laboratory-designed instruments makes such monitors impractical for industrial utilization. Devices suitable for industrial use, such as an air pollution study where several gases are present, typically require complex methods of analysis, such as mass spectrometry.
One known method of monitoring binary gas mixtures is illustrated in my U.S. Pat. No. 5,060,506. U.S. Pat. No. 5,060,506 recognizes that the velocity of sound through a gas varies with the gas composition and with temperature. A gas mixture is passed through a sample tube within which ultrasound waves travel in successive bursts of pulses at the resonant frequency of a transmitter/receiver pair. Between pulse bursts is a quiescent time period having a duration long enough to dissipate transients, so that standing waves do not form. A resulting voltage, which is proportional to the transit time of the ultrasound through the gas mixture, as well as to temperature error, is first corrected for gas temperature and then is available to measure the gas composition. It can also be compared with adjustable reference voltages to trigger high and low alarms.
More specifically, U.S. Pat. No. 5,060,506 teaches transmission of a brief burst of ultrasound energy through the body of a transducer at the resonant frequency common to both the transmitter and receiver elements of the transducer. A following quiescent period is sufficiently long to allow for dissipation of unwanted excess acoustical energy. A pulse transmitted from the transmitter to the receiver is selected electronically for analysis of transit time, long before standing waves can be established. The transit time analog value is stored during the quiescent period, and then up-dated by the subsequent pulse burst.
The teachings of my U.S. Pat. No. 5,060,506 are highly useful in the analysis of binary gas mixtures and, particularly, for monitoring gas purity from oxygen concentrators for respiratory care. Although the device and method described in my U.S. Pat. No. 5,060,506 is highly useful for its intended purposes, it is subject to some zero error caused by natural non-linearities in temperature effects and mechanical temperature coefficients within the transducer assembly. Another source of error is different specific heat ratios of various gases, but this is sufficiently small that it can generally be ignored. In addition to temperature-related errors is difference in the velocity of the flowing gases, which adds to the velocity of sound. At five liters per minute maximum in oxygen concentrators, the error is small enough to be ignored, but in many industrial applications, a closer tolerance is needed than is readily available with the device of my U.S. Pat. No. 5,060,056. Avoidance of routine calibration is also a prerequisite for most industrial applications.
Since acoustic monitoring of binary gases is affected by temperature and velocity changes to the gas, the need exists for a binary gas monitoring device that continuously compensates itself for changes in temperature and flow rate, thereby preventing repeated calibration by the user. The need also exists for such a device that is inexpensive and easy to manufacture, and does not require an expensive high-frequency system clock for operation. The need further exists for such a gas monitoring device that is operable at close tolerances to monitor even slight changes in composition of a binary gas. The present invention overcomes the drawbacks of the prior art and fills the foregoing and other needs.
A particular need addressed by the present invention is in gas generation and processing. One example is in the generation of ozone for water purification, odor elimination, bleaching, etc. Air, oxygen, or a mixture of both (air can be regarded as a single gas of molecular weight 29) is fed into a corona discharge tube and some percentage of the oxygen (O.sub.2) introduced is converted to ozone (O.sub.3). The entering O.sub.2 is the reference gas, the exiting O.sub.3 the sample. Another is in membrane separation of nitrogen from air for food storage, in which oxygen is a contaminant. In these two examples, flow rates into and out of the process are equal. A third example is in monitoring the purity of a sample gas against a standard gas which may be flowing at a different rate or not flowing at all. The error that can be caused by such unequal flow rates is corrected most easily in the transducer design, which will be described below.